A wavelength division multiplexing (WDM) optical transmission scheme of multiplexing a plurality of optical signals having different wavelengths in a single optical fiber and transmitting information is an very effective technique for implementing high capacity optical fiber communication.
A reconfigurable optical add drop multiplexer (ROADM) or a wavelength selective switch (WSS) is a type of an optical switch arranged at each node in an optical fiber network in which a WDM signal is transmitted. The ROADM is a general term for techniques, devices, or apparatuses used to split (drop) only a signal of a necessary wavelength from a WDM signal mainly transmitted through an optical fiber and receives the split signal or to add a wavelength signal transmitted from a current node to a WDM signal. The WSS is a WDM signal switching technique to which advanced wavelength selecting and routing functions are added, and can select an arbitrary number of signal wavelengths from a WDM signal in an input optical fiber line and output the selected arbitrary number of signal wavelengths to any one of an arbitrary number of (a plurality of) output fibers or extract optical signals of an arbitrary wavelength from WDM signals in a plurality of input fibers and output the extracted optical signals to an output fiber.
The optical switching technique attracts public attention due to its convenience as a technique essential for flexibly setting a wavelength line according to the growth of an optical network or a change in traffic in each region and implementing a ring or mesh network which is high in fault tolerance or traffic accommodation efficiency. Hereinafter, in this specification, the ROADM technique and the WSS technique are collectively referred to as a “wavelength selective switch (WSS).”
The representative scheme of WDM optical transmission has been described above. Next, a WDM technique to which the WSS technique is applied will be described.
FIG. 1 is a diagram illustrating an example of a wavelength spectrum of a WDM signal. FIG. 1(1) illustrates an optical signal spectrum of WDM according to a related art. Each of ch1 to ch6 is a channel of an optical signal modulated by a high-speed information signal of 2.5 to 100 gigabits/second or the like, and each optical signal is a spectrum having a width of several to several tens of 10 GHz by high-speed modulation. Each of λ1 to λ6 is a center wavelength (or a center frequency) of each channel, and for an absolute wavelength and spacing thereof, an arrangement standardized as a wavelength grid by the International Telecommunication Union (ITU) is used.
In FIG. 1, G represents spacing of a wavelength grid. As a general wavelength grid, for example, those having spacing of 200 GHz, 100 GHz, 50 GHz, or 25 GHz are used. Since an impairment is caused due to interference when a spectrum of an optical channel overlaps a spectrum of an adjacent channel, each optical channel has a grid spacing G larger than a spectrum width (b4 or b5 in FIG. 1) of each signal, and a sufficient gap (for example, about 10 to 20% of channel spacing) is given between channels. Particularly, when wavelength splitting is performed by the WSS or the ROADM above, a wavelength is split using this gap portion.
Meanwhile, an amount of information transmitted in an optical network gradually increases yearly, and in recent high-speed optical fiber transmission, the application of a ultrahigh-speed signal in which a transmission rate per wavelength exceeds 100 gigabits/second has been under review. Further, since there is a limitation to a wavelength band that can be used for long-distance transmission, the application of a high-efficiency modulation format such as optical multilevel transmission of cramming a lot of information in a limited wavelength (frequency) band and thus improving efficiency of spectral usage has been under review. As an example of this technique, for example, there is a technique disclosed in NPL 1.
In NPL 1, information of 8 bits per symbol is crammed by multilevel (or sixteen-level) modulation and polarization multiplexing technique, and high-efficiency transmission is performed using a high-sensitivity coherent homodyne receiver having wavelength/polarization selectivity.
FIGS. 1(2) to 1(4) illustrate exemplary wavelength spectrums of optical modulation formats in which efficiency of spectral usage is further improved than in FIG. 1(1). A normal optical signal has a rounded signal spectrum (for example, a raised cosine type) in which intensity is high at the center of a wavelength and intensity decreases as a distance from the center increases as illustrated in FIG. 1(1). In this spectrum, usage efficiency of a frequency component away from the center in a spectrum width (b4 in the case of ch4 in FIG. 1) of a signal is known to decrease. Ideally, it is possible to cram more information in a specified bandwidth using an optical signal of a rectangular spectral shape as illustrated in FIG. 1(2). Further, it is possible to improve the efficiency of spectral usage up to the limit by eliminating a gap between channels.
Practically, it is difficult to generate an optical signal of a rectangular spectral shape perfectly, but as a technique of obtaining a signal close to a rectangle in a pseudo manner as illustrated in FIG. 1(3), there is a technique of applying Nyquist filtering to a modulation signal. Further, it is possible to improve the efficiency of spectral usage by setting a plurality of subcarriers in a certain bandwidth of a channel, performing optical multilevel modulation on each of the subcarriers, and keeping spectrum intensity in a bandwidth constantly as illustrated in FIG. 1(4). The same effect is obtained by orthogonal frequency division multiplexing (OFDM) (FIG. 1(5)) of generating a plurality of low-speed orthogonal carriers using digital signal processing. Although any of such modulation is used, it is possible to further improve information transmission efficiency by densely allocating channels at minimum spacing between adjacent wavelength channels.
The basic scheme of performing wavelength division multiplexing (WDM) on an optical signal has been described above. Next, splitting of a WDM signal using a WSS according to a related art will be described.
FIG. 2 is a diagram illustrating a configuration of a WSS node 100 according to a related art. The WSS node 100 is a network node in which a WSS of a related art is mounted. The WSS node 100 has a function of arbitrarily splitting a WDM signal input from a single input optical fiber line 101 for each wavelength channel and outputting the split WDM signals to two output optical fiber lines 103-1 and 103-2. Generally, a pair of an uplink and a downlink is used as an optical fiber line, but the present invention will be described in view of only signal transmission of one direction since no problem related to directivity particularly occurs.
FIG. 3 is a diagram illustrating a relation between an optical signal and a transmission band of the WSS node 100 according to the related art. A WDM signal 102 input from the input optical fiber line 101 is assumed to include 6 wavelength signals of channels 1 to 6 (wavelengths λ1 to λ6), and signal spectrum thereof are illustrated in FIG. 3(1). In the example illustrated in FIG. 3, in the WDM signal 102, signals of the channels 3 and 4 are assumed to be split and output to the output optical fiber line 103-1, and optical signals of the channels 5 and 6 are assumed to be split and output to the output optical fiber line 103-2.
The WSS of the related art is configured with a liquid crystal MEMS switch, a spatial diffraction grating, and the like, and has a function of changing transmittance to each output fiber with wavelength spacing corresponding to a wavelength grid using a border (a grid border) of a wavelength grid as a border line. In this regard, for the output optical fiber line 103-1, a transmission band is set as illustrated in FIG. 3(2) so that input light transmits in the grids (corresponding to bands B4 and B5) of the wavelengths λ3 and λ4. As a result, the wavelength channels 3 and 4 can be output to the output optical fiber line 103-1 as an output WDM signal 104-1 as illustrated in FIG. 3(3).
Similarly, for the output optical fiber line 103-2, a transmittance is set as illustrated in FIG. 3(4) so that input light transmits in the grids of the wavelengths λ5 and λ6. As a result, the wavelength channels 5 and 6 can be output to the output optical fiber line 103-2 as an output WDM signal 104-2.
An effect of splitting the channel 4 and the channel 5 which are channels adjacent to each other is obtained by an action between steeply inclined portions (shoulder portions) of transmittance (FIG. 3(2)) for transmitting the channel 4 and transmittance (FIG. 3(4)) for transmitting the channel 5. Thus, in order to properly splitting a wavelength, the inclined portion of the transmittance needs to exactly overlap the grid border.
FIG. 3(6) illustrates a state in which the transmittance of FIG. 3(2) and the transmittance of and FIG. 3(3) are displayed to overlap. V4 represents a width of transmittance to the output optical fiber line 103-1 from the center wavelength (λ4) of the wavelength channel 4 which is measured at the wavelength channel 5 side. V5 represents the width of transmittance to the output optical fiber line 103-2 from the center wavelength (λ5) of the wavelength channel 5 which is measured at the wavelength channel 4 side. A measurement target is a range of a portion (loss of about 0.5 to 1 dB) in which a characteristic shape of a transmission band is almost flat.
In FIG. 3(6), each of the two transmittances has an inclined portion between both channels, and a sum of V4 and V5 is smaller than channel spacing (λ5−λ4) between both channels. Thus, in the WSS of the related art, the flat portions (the ranges of V4 and V5 indicated by arrows) of the transmission bands of the optical signals of the adjacent channels do not overlap.